diff --git a/Makefile b/Makefile
index bc1f0cb82ef6..f5026f769945 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@
 # buildworld          - Rebuild *everything*, including glue to help do
 #                       upgrades.
 # installworld        - Install everything built by "buildworld".
-# world               - buildworld + installworld.
+# world               - buildworld + installworld, no kernel.
 # buildkernel         - Rebuild the kernel and the kernel-modules.
 # installkernel       - Install the kernel and the kernel-modules.
 # installkernel.debug
@@ -37,7 +37,8 @@
 # system, the simple instructions are:
 #
 # 1.  `cd /usr/src'  (or to the directory containing your source tree).
-# 2.  `make world'
+# 2.  Define `HISTORICAL_MAKE_WORLD' variable (see README).
+# 3.  `make world'
 #
 # For individuals wanting to upgrade their sources (even if only a
 # delta of a few days):
diff --git a/README b/README
index 87cfa1754903..674d93ebe0a9 100644
--- a/README
+++ b/README
@@ -11,8 +11,13 @@ The Makefile in this directory supports a number of targets for
 building components (or all) of the FreeBSD source tree, the most
 commonly used one being ``world'', which rebuilds and installs
 everything in the FreeBSD system from the source tree except the
-kernel, the kernel-modules and the contents of /etc.  The
-``buildkernel'' and ``installkernel'' targets build and install
+kernel, the kernel-modules and the contents of /etc.  The ``world''
+target should only be used in cases where the source tree has not
+changed from the currently running version.  See:
+http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/makeworld.html
+for more information, including setting make(1) variables.
+
+The ``buildkernel'' and ``installkernel'' targets build and install
 the kernel and the modules (see below).  Please see the top of
 the Makefile in this directory for more information on the
 standard build targets and compile-time flags.